Some tidbits about weird behavior and/or performance characteristics in PHP, in particular relating to object properties.

1. PHP Language Trivia
Nikita Popov
(nikic)

2. Object properties

3. Object properties
Name mangling

4. class Test {
public $pub = 1;
protected $prot = 2;
private $priv = 3;
}
$obj = new Test;
$arr = (array) $obj;
var_dump($arr);

5. class Test {
public $pub = 1;
protected $prot = 2;
private $priv = 3;
}
$obj = new Test;
$arr = (array) $obj;
var_dump($arr);
array(3) {
["pub"]=>
int(1)
["*prot"]=>
int(2)
["Testpriv"]=>
int(3)
}

6. class Test {
public $pub = 1;
protected $prot = 2;
private $priv = 3;
}
$obj = new Test;
$arr = (array) $obj;
var_dump($arr);
var_dump($arr["*prot"]);
// Notice: Undefined index: *prot
array(3) {
["pub"]=>
int(1)
["*prot"]=>
int(2)
["Testpriv"]=>
int(3)
}

7. class Test {
public $pub = 1;
protected $prot = 2;
private $priv = 3;
}
$obj = new Test;
$arr = (array) $obj;
var_dump($arr);
var_dump($arr["0*0prot"]);
// int(2)
array(3) {
["pub"]=>
int(1)
["0*0prot"]=>
int(2)
["0Test0priv"]=>
int(3)
}

8. Why name mangling?

9. class A {
private $prop = 'A';
public function getPropA() { return $this->prop; }
}
class B extends A {
protected $prop = 'B';
public function getPropB() { return $this->prop; }
}
class C extends B {
public $prop = 'C';
public function getPropC() { return $this->prop; }
}
$obj = new C;
var_dump($obj->getPropA()); // string(1) "A"
var_dump($obj->getPropB()); // string(1) "C"
var_dump($obj->getPropC()); // string(1) "C"

10. class A {
private $prop = 'A';
public function getPropA() { return $this->prop; }
}
class B extends A {
protected $prop = 'B';
public function getPropB() { return $this->prop; }
}
class C extends B {
public $prop = 'C';
public function getPropC() { return $this->prop; }
}
$obj = new C;
var_dump($obj->getPropA()); // string(1) "A"
var_dump($obj->getPropB()); // string(1) "C"
var_dump($obj->getPropC()); // string(1) "C"
Refer to same property

11. class A {
private $prop = 'A';
public function getPropA() { return $this->prop; }
}
class B extends A {
protected $prop = 'B';
public function getPropB() { return $this->prop; }
}
class C extends B {
public $prop = 'C';
public function getPropC() { return $this->prop; }
}
$obj = new C;
var_dump($obj->getPropA()); // string(1) "A"
var_dump($obj->getPropB()); // string(1) "C"
var_dump($obj->getPropC()); // string(1) "C"
Refer to same property
Private property is independent

12. class A {
private $prop = 'A';
public function getPropA() { return $this->prop; }
}
class B extends A {
protected $prop = 'B';
public function getPropB() { return $this->prop; }
}
class C extends B {
public $prop = 'C';
public function getPropC() { return $this->prop; }
}
$obj = new C;
var_dump((array) $obj);
Refer to same property
Private property is independent

13. class A {
private $prop = 'A';
public function getPropA() { return $this->prop; }
}
class B extends A {
protected $prop = 'B';
public function getPropB() { return $this->prop; }
}
class C extends B {
public $prop = 'C';
public function getPropC() { return $this->prop; }
}
$obj = new C;
var_dump((array) $obj);
array(2) {
["0A0prop"]=> string(1) "A"
["prop"]=> string(1) "C"
}
Refer to same property
Private property is independent

14. Object can have multiple properties
with same name
Name mangling ensures
unique property names

15. No reason to expose this internal detail …

16. No reason to expose this internal detail …
… but libraries rely on it now
to access private properties

17. Object properties
Integer property names

18. $array = [];
$array[123] = "foo";
$array["123"] = "bar";
var_dump($array);
array(1) {
[123]=>
string(3) "bar"
}

19. $array = [];
$array[123] = "foo";
$array["123"] = "bar";
var_dump($array);
array(1) {
[123]=>
string(3) "bar"
}
$object = new stdClass;
$object->{123} = "foo";
$object->{"123"} = "bar";
var_dump($object);
object(stdClass)#1 (1) {
["123"]=>
string(3) "bar"
}

20. $array = [];
$array[123] = "foo";
$array["123"] = "bar";
var_dump($array);
array(1) {
[123]=>
string(3) "bar"
}
$object = new stdClass;
$object->{123} = "foo";
$object->{"123"} = "bar";
var_dump($object);
object(stdClass)#1 (1) {
["123"]=>
string(3) "bar"
}
Normalize to int Normalize to string

21. $array = [];
$array[123] = "foo";
$array["123"] = "bar";
var_dump($array);
array(1) {
[123]=>
string(3) "bar"
}
$object = new stdClass;
$object->{123} = "foo";
$object->{"123"} = "bar";
var_dump($object);
object(stdClass)#1 (1) {
["123"]=>
string(3) "bar"
}
Normalize to int Normalize to string
What happens if we mix both?

22. $array = [123 => "foo"];
$object = (object) $array;
var_dump($object->{123});
// Notice: Undefined property: stdClass::$123
var_dump($object->{"123"});
// Notice: Undefined property: stdClass::$123

23. $array = [123 => "foo"];
$object = (object) $array;
var_dump($object->{123});
// Notice: Undefined property: stdClass::$123
var_dump($object->{"123"});
// Notice: Undefined property: stdClass::$123
var_dump($object);
object(stdClass)#1 (1) {
[123]=>
string(3) "foo"
}

24. $array = [123 => "foo"];
$object = (object) $array;
var_dump($object->{123});
// Notice: Undefined property: stdClass::$123
var_dump($object->{"123"});
// Notice: Undefined property: stdClass::$123
var_dump($object);
object(stdClass)#1 (1) {
[123]=>
string(3) "foo"
}
Unnormalized integer property name

25. $object = new stdClass;
$object->{123} = "foo";
$array = (array) $object;
var_dump($array[123]);
// Notice: Undefined offset: 123
var_dump($array["123"]);
// Notice: Undefined offset: 123

26. $object = new stdClass;
$object->{123} = "foo";
$array = (array) $object;
var_dump($array[123]);
// Notice: Undefined offset: 123
var_dump($array["123"]);
// Notice: Undefined offset: 123
var_dump($array);
array(1) {
["123"]=>
string(3) "foo"
}
Unnormalized integral string key

27. Fixed in PHP 7.2!
Now integer keys are renormalized
on array->object and object->array casts

28. $array = [123 => "foo"];
$object = (object) $array;
var_dump($object->{123});
string(3) "foo"
$object = new stdClass;
$object->{123} = "foo";
$array = (array) $object;
var_dump($array[123]);
string(3) "foo"

29. Object properties
Memory usage

30. $array = [
"key1" => 1,
"key2" => 2,
// ...
];
class Value {
public $key1;
public $key2;
}
$object = new Value;
$object->key1 = 1;
$object->key2 = 2;
vs.

31. 0
100
200
300
400
500
600
700
800
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Memoryusage(bytes)
Number of properties/keys
Array Array (real) Object Object (real)

32. 0
1
2
3
4
5
6
7
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Arraysize/objectsize
Number of properties/keys
Ratio Ratio (real)

33. Optimized for different usecases
Objects: Good for fixed set of keys
Arrays: Good for dynamic set of keys

34. Class entry
Property 0
Property 1
…
Object

35. Class entry
Property 0
Property 1
…
Object
Contains [property name => property offset] map

36. Class entry
Properties array
Property 0
Property 1
…
Object
Contains [property name => property offset] map
[property name => property value] map,
used if there are dynamic properties

37. Class entry
Properties array
Property 0
Property 1
…
Object
Contains [property name => property offset] map
[property name => property value] map,
used if there are dynamic properties
Arrays:
• Store keys (and hashes) explicitly
• Always have power of two size (8, 16, …)
for faster insertions

38. class Value {
public $x;
}
$obj = new Value;
// $obj size: 56 bytes
foreach ($obj as $k => $v) { }
// $obj size: 432 bytes

39. class Value {
public $x;
}
$obj = new Value;
// $obj size: 56 bytes
foreach ($obj as $k => $v) { }
// $obj size: 432 bytes
Forces creation of properties array

40. class Value {
public $x;
}
$obj = new Value;
// $obj size: 56 bytes
foreach ($obj as $k => $v) { }
// $obj size: 432 bytes
Forces creation of properties array
… no way to get rid of it afterwards

41. // PhpParser node iteration
$names = $node->getSubNodeNames();
foreach ($names as $name) {
$value = $node->$name;
}

42. // PhpParser node iteration
$names = $node->getSubNodeNames();
foreach ($names as $name) {
$value = $node->$name;
}
Dynamic lookup is slow, but this
avoids large memory usage increase

43. Object properties
Magic get & set

44. Direct property access baseline
getProperty() method 2.2x slower
__get() magic 6.0x slower

45. Direct property access baseline
getProperty() method 2.2x slower
__get() magic 6.0x slower
Userland  internal  userland is slow

46. class Test {
public function __get($name) {
return $this->$name;
}
}

47. class Test {
public function __get($name) {
return $this->$name;
}
} Does not recurse into __get()
Will access property directly

48. class Test {
public function __get($name) {
return $this->$name;
}
} Does not recurse into __get()
Will access property directly
Recursion guards are property name + accessor type specific

49. class Test {
public function __get($name) {
return $this->$name;
}
} Does not recurse into __get()
Will access property directly
Recursion guards are property name + accessor type specific
In __get("foo"):
• $this->foo will access property
• $this->bar will call __get("bar")
• $this->foo = 42 will call __set("foo", 42)

50. __get("foo")
__get("bar")
__set("bar", 42)
Recursion guards:
[
"foo" => GET,
"bar" => GET|SET,
]

51. __get("foo")
__get("bar")
[
"foo" => GET,
"bar" => GET,
]
Recursion guards:

52. __get("foo")
[
"foo" => GET,
"bar" => 0,
]
Recursion guards:

53. [
"foo" => 0,
"bar" => 0,
]
Recursion guards:

54. [
"foo" => 0,
"bar" => 0,
]
Recursion guards:
Never cleaned up

55. [
"foo" => 0,
"bar" => 0,
]
Recursion guards:
Never cleaned up
PHP 7.1: Recursion guard array not used if
magic accessors used only for one property at a time

56. Object properties
Unset properties

57. class Test {
public $prop;
}
$obj = new Test;
unset($obj->prop);
var_dump($obj->prop);
// Notice: Undefined property: Test::$prop

58. class Test {
public $prop;
}
$obj = new Test;
unset($obj->prop);
var_dump($obj->prop);
// Notice: Undefined property: Test::$prop
Once unset, __get() will be called on access
-> Lazy initialization

59. class Test {
public $prop;
public function __construct() {
unset($this->prop);
}
public function __get($name) {
echo "__get($name)n";
$this->$name = "init";
return $this->$name;
}
}
$obj = new Test;
var_dump($obj->prop);
var_dump($obj->prop);

60. class Test {
public $prop;
public function __construct() {
unset($this->prop);
}
public function __get($name) {
echo "__get($name)n";
$this->$name = "init";
return $this->$name;
}
}
$obj = new Test;
var_dump($obj->prop);
var_dump($obj->prop);
Calls __get()
Does not call __get()

61. Scoped calls

62. Foo::bar()
Static method call … or is it?

63. class A {
public function method() {
/* ... */
}
}
class B extends A {
public function method() {
parent::method();
/* ... */
}
}

64. class A {
public function method() {
/* ... */
}
}
class B extends A {
public function method() {
A::method();
/* ... */
}
}

65. class A {
public function method() {
/* ... */
}
}
class B extends A {
public function method() {
A::method();
/* ... */
}
}
Scoped instance call:
Call A::method() with current $this

66. class A {
public function method() { /* ... */ }
}
class B extends A {
public function method() { /* ... */ }
}
class C extends B {
public function method() {
A::method();
/* ... */
}
}
Can also call grandparent method

67. class A {
public function method() {
echo 'A::method with $this=' . get_class($this) . "n";
}
}
class B /* does not extend A */ {
public function method() {
A::method();
}
}
(new B)->method();

68. class A {
public function method() {
echo 'A::method with $this=' . get_class($this) . "n";
}
}
class B /* does not extend A */ {
public function method() {
A::method();
}
}
(new B)->method();
// PHP 5: A::method with $this=B (+ deprecation)

69. class A {
public function method() {
echo 'A::method with $this=' . get_class($this) . "n";
}
}
class B /* does not extend A */ {
public function method() {
A::method();
}
}
(new B)->method();
// PHP 5: A::method with $this=B (+ deprecation)
// PHP 7.0: Undefined variable: this
// PHP 7.1: Error: Using $this when not in object context

70. class Test {
public function __call($name, $args) {
echo "__call($name)n";
}
public static function __callStatic($name, $args) {
echo "__callStatic($name)n";
}
public function doCall() {
Test::foobar();
}
}
Test::foobar();
(new Test)->doCall();

71. class Test {
public function __call($name, $args) {
echo "__call($name)n";
}
public static function __callStatic($name, $args) {
echo "__callStatic($name)n";
}
public function doCall() {
Test::foobar();
}
}
Test::foobar(); // __callStatic(foobar)
(new Test)->doCall();

72. class Test {
public function __call($name, $args) {
echo "__call($name)n";
}
public static function __callStatic($name, $args) {
echo "__callStatic($name)n";
}
public function doCall() {
Test::foobar(); // __call(foobar)
}
}
Test::foobar(); // __callStatic(foobar)
(new Test)->doCall();

73. Static Closures

74. class Test {
public function __construct() {
$this->fn = function() {
/* $this can be used here */
};
}
}

75. class Test {
public function __construct() {
$this->fn = static function() {
/* $this CANNOT be used here */
};
}
}

76. class Test {
public function __construct() {
$this->fn = static function() {
/* $this CANNOT be used here */
};
}
} Without static:
• Closure references $this
• $this->fn references Closure

77. class Test {
public function __construct() {
$this->fn = static function() {
/* $this CANNOT be used here */
};
}
} Without static:
• Closure references $this
• $this->fn references Closure
 Cycle causes delayed GC